In a standard memory module (also called DIMM-dual inline memory module), the silicon area as well as the convective surface of the substrate is large enough to keep the thermal challenges under control. Thus a conventional DIMM, compared to very low profile-DIMMs (VLP-DIMM)) does not demand stringent air flow or thermally-centric design. Increasing the capacity of VLP-DIMM requires multiple chip stacks, thereby increasing the thickness of a module in the direction detrimental to air-flow. When the cross-sectional area available for air flow is at premium within a blade, a thicker VLP-DIMM module creates undesirable reduction in airflow velocity because the impedance to airflow increases with a reduced cross sectional area. Under this restricted condition, enhancement to cooling of a VLP-DIMM or any other memory module becomes important.
Personal computers, work stations and blade servers are designed to accommodate field replaceable memory modules. Referring to FIG. 1 there is shown a typical memory module. The memory modules, also referred to as DIMMS (dual inline memory modules) typically have an industry standard form factor as shown in FIG. 1A. FIG. 1A shows a DIMM module 100 with the basic components of: a printed circuit board 110, memory chips 120, pins 150, and notches 140. Notches 140 line up with the socket where the DIMM 100 is to be inserted. Also shown are the x, y, and z forces acting on the DIMM 100. FIG. 1B shows a side view of the DIMM 100 of FIG. 1A. FIG. 1C shows a top view of the DIMM 100 and FIG. 1D shows a side view of the DIMM 100 affixed to a computer component. In this DIMM 100, the corresponding electrical connectors and memory communication protocol are also standardized.
The memory capacity offered by the industry tends to increase with time. For computers that require larger memory capacity, multiple memory modules are used. Since multiple modules require as many connectors, they require a proportional amount of board space. Therefore a constraint is self-imposed on the number of connectors by a designer of a computer. This, however, imposes a limit on memory capacity which determines overall system performance. In addition to memory capacity, the height of a memory module that dictates the slimness of a server blade is a competitive differentiator. A memory product, sometimes referred to as VLP-DIMMs (Very Low Profile), caters to the needs of the slim blade server industry.
Slim blade servers are attractive for customers requiring large computing power within a limited rack space. Typically about 14 slim blade servers are packed in a 19″ rack compared to 10 blades with standard DIMMs. Since the volume available for electronic components and airflow space are correspondingly reduced in a slim blade server, new design challenges are encountered. The heat dissipation in electronic components more or less remains the same in both servers, but the surface area available for heat transfer is reduced and the resistance to airflow is increased. This trend complicates the management of heat in a slim blade server.
Since the planar area available in the X-Y plane, as shown in FIG. 1A, for a memory module is reduced in a VLP-DIMM, manufacturers opt to stack several layers of memory chips in order to gain higher capacity. FIGS. 2A, 2B, and 2C illustrate the thermal path of a two-stack design. The heat generated in a silicon chip is conducted through the substrate 210 and solder connections 250, and is eventually dissipated into the air 270 through convection, one of the thermal transport mechanisms. In a standard DIMM 100 the silicon area as well as the convective surface of the substrate is large enough to keep the thermal challenges under control. Thus a conventional DIMM 100 does not require increased local air flow or additional heat dissipating structures as do the VLP-DIMMs.
Increasing the capacity of a VLP-DIMM requires multiple chip stacks, thereby increasing the thickness of a module in the Z-direction as illustrated in FIG. 1A. When the cross-sectional area available for air flow is at a premium within a blade, a thicker VLP-DIMM module creates an undesirable reduction in airflow velocity because the impedance to airflow increases with a reduced cross sectional area. Under this restricted condition, enhancement to cooling of a VLP-DIMM or any other memory module becomes important.
Therefore, there is a need for a better cooling structure for VLP-DIMM modules to overcome the shortcomings of the prior art.